"Your Stock Exchange has performed an illegal operation, and must be shut down."

     It "glitched."  The New York Stock Exchange's computer system...freaked out.  Froze.  And on the same day as an airline's system choked, too.

     ...Instant OMG, with everyone from the Russians to the President to blame.  "Vast right-wing conspiracy" -- ey, Mrs. Clinton? -- didn't get drug in, but give the fringes time; the Chinese certainly did, 'cos what's more fun than to cripple the number one market for your mass-produced junk?

    While I have come to approve of the long-standing American tradition (it starts with POTUS #1) of suspecting the Chief Executive of being a sinister spider at the center of a vast and nefarious web of conspiratorial intrigue, I suspect we generally tend to over-estimate the competence of Presidents.  Oh, they may well all be just as wicked as their opponents claim, especially the last half-dozen -- it's ability they lack, or they'd all be living like Bond villains long before they got offered the apartment over the shop at 1600 Pennsylvania Avenue.

     Other actors -- Anonymous would have signed their work (anonymously, of course), 4chan only wish they could manage such a stunt and the various sousveillance troupes would have had a lot more fun with it.  Mr. Putin's government?  Maybe -- maybe -- they could, but what's in it for them? Follow the money, and-- Nobody profited.  "Trial run," some mutter, but with cyberweaponry, you don't tip your hand that way: revealing an exploitable vulnerability is the exact equivalent to putting in a request to have it fixed, only with a higher priority.*

     Nope, I think what we have here are plain old "oops" type failures, writ large -- ill-considered software updates, hardware failures, whatever.  When you have very complex systems, which is where we're at, the potential for crashes goes up; and every so often, you get a black swan event -- or, worse, a blue moon: a cascading failure or an unexpected single point of failure takes a linked system right down.

     Boilers still sometimes explode, even with all the safety valves and cut-outs and overpressure alarms.  You don't notice it; there aren't so many around, the technology is very mature so booms are less and less frequent and there is plenty of redundancy.  Computers and computer networks are considerably more common than and about as advanced as steam boilers were a century ago -- and sometimes, despite all efforts to prevent it happening, they crash.
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* Those of us with enough gray in our hair remember when That's How It Was Done: when computers were big time-shared mainframes slightly dumber than the smartphone in your pocket, crashing the system was sometimes the only way to get problems fixed and a clever console op would offer forgiveness for confessors.